1. Field of the Invention
The present invention relates to an optical-writing control device, an image forming apparatus, and a method of controlling an optical writing device and, more particularly, to a structure of a pattern to be drawn for correction of an image drawing position.
2. Description of the Related Art
In recent years, there are increasing tendencies to further digitization of information. Thus, image processing apparatuses such as printers and facsimiles for use in outputting digitized information and scanners for use in digitizing documents have become indispensable equipment. Such an image processing apparatus is typically configured as a multifunction peripheral (MFP) having an image capturing function, an image forming function, a communication function, and/or the like and is usable as a printer, a facsimile, a scanner, and/or a copier.
Among such image processing apparatuses, electrophotographic image forming apparatuses are widely used in outputting digitized documents. An electrophotographic image forming apparatus forms an electrostatic latent image by exposing a photosensitive element to light, develops the electrostatic latent image using a developing agent such as toner to form a toner image, and transfers the toner image onto a sheet to thereby produce a printout.
Such an electrophotographic image forming apparatus performs adjustment so that an image is formed in an appropriate area on a sheet by synchronizing a timing of exposing the photosensitive element to light to draw an electrostatic latent image with a timing of conveying the sheet. Moreover, a tandem image forming apparatus that forms a multiple-color image using a plurality of photosensitive elements adjusts a timing of exposing each of the photosensitive elements for the respective colors to light so that images of the respective colors developed on the photosensitive elements are properly overlaid on one another. Hereinafter, these adjustments are collectively referred to as positional deviation correction.
Concrete methods for performing such positional deviation correction as described above include a mechanical-adjustment-based method and an image-processing-based method. The mechanical-adjustment-based method adjusts positional relationship between a light source for use in exposure of the photosensitive element and the photosensitive element. The image-processing-based method adjusts an image to be formed according to positional deviation, thereby causing the image to be eventually formed at an opportune position. The image-processing-based method is performed by drawing and reading a correction pattern, and making correction based on difference between a timing determined from design and a timing when the pattern is actually read so that an image is formed at a desired position.
Regarding the image-processing-based method, for example, a technique for increasing reading accuracy of a sensor that reads a correction pattern is proposed (see, for example, Japanese Laid-open Patent Application No. 2004-069767. According to the technique disclosed in Japanese Laid-open Patent Application No. 2004-069767, a correction pattern that is larger than a read area of the reading sensor to have a margin or, more specifically, a correction pattern that is large enough to permit reading even when positional deviation should occur, is drawn. After making correction based on this correction pattern, correction is made second time using a correction pattern that is drawn in a size that depends on the read area of the reading sensor. This technique allows excluding, from the correction made second time, an influence of diffused light reflected from the margin portion of the correction pattern, making highly-accurate correction possible.
When a correction pattern is drawn in a size that depends on a read area of a reading sensor as disclosed in Japanese Laid-open Patent Application No. 2004-069767, the size of the drawn pattern simply becomes small. Accordingly, an advantage of decrease in toner consumption can also be obtained. To enhance this advantage, it is desired to minimize the correction pattern drawn with the margin to the read area of the reading sensor.
Meanwhile, when an area where a correction pattern is drawn has no margin to the read area of the reading sensor, possibility that the correction pattern is not detected properly increases. Anomalous detection of the correction pattern leads to the disadvantage that correction of a parameter that is to be corrected based on the correction pattern is not normally performed, and normal device operation is prevented. Thus, there is a trade-off between the reduction in toner consumption and accuracy of device operation owing to normal detection of a correction pattern described above.
Example structures of the correction pattern described above include a structure in which the correction pattern includes four patterns: a registration-timing correction pattern; a color registration error correction pattern; a main-scanning correction pattern; and a density correction pattern. Among the patterns, the registration-timing correction pattern is used not only for a purpose of correcting a drawing position of an entire image in the main-scanning direction, that is, correcting a registration timing, but also for a purpose of correcting a timing of detecting the density correction pattern.
If a registration timing deviates because the registration timing is not corrected properly, a position on a sheet where the image is formed deviates. If, as a result, the image is transferred onto a leading end of the sheet, the sheet can undesirably stick to a fixing roller when the sheet is conveyed to a fixing unit, resulting in paper jam.
If the timing of detecting the density correction pattern is not corrected properly and a detection timing deviates, the density correction pattern is not detected properly, resulting in erroneous density correction. In short, when the registration-timing correction pattern drawn for the multiple purposes is not detected properly, a plurality of inconveniences will be caused, which is highly disadvantageous.
In light of these, there is a need to balance reduction in amount of toner to be consumed to draw correction patterns and a degree of accuracy in device operation.